add an overview of relevant riak_core modules

At least those already being used in the tutorial: riak_core_ring, riak_core_apl, etc.

Something similar is done in http://www.erlang-factory.com/upload/presentations/294/MasterlessDistributedComputingwithRiakCore-RKlophaus.pdf

```
- 0. Riak Core overview
+ -1. Consistent hashing overview
+ 0. Riak core module overview
```
